Unit 2_DC Drive
By
Dr. Amruta Pattnaik
Ward Leonard Method of Speed Control
The arrangement consists of a seperately excited
d.c motor M whose speed is to be controlled. G
is a seperately excited dc generator driven by a
prime mover, usually an induction motor. Motor
generator set converts ac to dc
Block diagram of a Ward-Leonard scheme
employing an ac motor for driving dc generator
Ward Leonard Method Procedure
• DC motor field is excited by a dc source. Now, the generator field
flux is kept to a low value to produce a small output voltage, to
limit armature current through motor.
• A change in the generated field current varies the applied voltage
to the motor and therefore, the speed is changed. So, motor
speed is controlled merely by changing generator field current.
• Thus by varying the voltage applied to the motor from a small
value to rated value, speeds below rated speed can be obtained.
• Further, by varying motor field and keeping voltage constant
speeds above base speed can be obtained.
• If you reverse generator field terminals, opposite polarity voltage
is generated by the generated and speed get reversed.
Advantages of Ward Leonard Method
No starting rheostat are used therefore less power wastage.
Accurate, wide range and bidirectional speeds are obtained easily.
Speed regulation is very good.
Motor direction is changed merely by changing generator field current.
Thus, It has inherent regenerative braking property.
Disdvantages of Ward Leonard System
The method is very costly as two extra machines (motor-generator set)
are required and requires more floor area.
Overall efficiency of the system is not sufficient especially if it is lightly
loaded due to large mechanical losses in three machines.
ward leonard system applications
Rolling mills, colliery winding motor and Lifts and elevators are some of
the examples employing ward leonard method of speed control.
Four Quadrant operation in Motor :
• Below Figure shows the polarities of the supply voltage V a, back emf
Eg, and armature current Ia for a separately excited motor.
Four Quadrants:
• the polarities of the supply voltage V a, back emf Eg, and armature current Ia for
a separately excited motor.
• In forward motoring (quadrant I), Va, Eg, and Ia are all positive. The torque and
speed are also positive in this quadrant.
• During forward braking (quadrant II), the motor runs in the forward direction
and the induced emf Eg continues to be positive. For the torque to be negative
and the direction of energy flow to reverse, the armature current must be
negative. The supply voltage Va should be kept less than Eg.
• In reverse motoring (quadrant III), Va, Eg, and Ia are all negative. The torque
and speed are also negative in this quadrant. To keep the torque negative and
the energy flow from the source to the motor, the back emf E g must satisfy the
condition | Va | > | Eg |. The polarity of Eg can be reversed by changing the
direction of field current or by reversing the armature terminals.
• During reverse braking (quadrant IV), the motor runs in the reverse direction.
Va, and Eg continue to be negative. For the torque to be positive and the energy
to flow from the motor to the source, the armature current must be positive.
The induced emf Eg must satisfy the condition | V a | < | Eg |.
Controlled Rectifier Fed DC Drives:
• Controlled Rectifier Fed DC Drives are used to
get variable dc voltage from an ac source of
fixed voltage.
• Controlled Rectifier Fed DC Drives are also
known as Static Ward-Leonard drives.
• All the Figure shows commonly used Controlled Rectifier Fed DC Drives
and quadrants in which they can operate on Va-Ia plane.
• As thyristors are capable of conducting current only in one direction, all
these rectifiers are capable of providing current only in one direction.
• Rectifiers of Figs. (a) and (c) provide control of dc voltage in either
direction and therefore, allow motor control in quadrants I and IV. They
are known as Fully Controlled Rectifiers.
• Rectifiers of Figs. (b) and (d) are called Half Controlled Rectifiers as they
allow dc voltage control only in one direction and motor control in
quadrant I only.
• For low power applications (up to around 10 kW) single-phase rectifier
drives are employed. For high power applications, three-phase rectifier
drives are used. Exception is made in traction where single phase drives
are employed for large power ratings.
Single Phase Fully Controlled Rectifier Control of DC Motor
The Single Phase Fully Controlled Rectifier Control of DC Motor is shown
in Fig. (a).
Motor is shown by its equivalent circuit.
Field supply is not shown.
When field control is required, field is fed from a controlled rectifier,
otherwise from an uncontrolled rectifier.
The ac input voltage is defined by
In a cycle of source voltage, thyristors T1 and T3 are given gate signals from α to π, and
thyristors T2 and T4 are given gate signals from (π + α) to 2π.
When armature current does not flow continuously, the motor is said to operate in
discontinuous conduction. When current flows continuously, the conduction is said to be
continuous.
In discontinuous conduction mode of Single Phase Fully Controlled
Rectifier Control of DC Motor, current starts flowing with the turn-on of
thyristors T1 and T3 at ωt = α.Motor gets connected to the source and its
terminal voltage equals vs. The current, which flows against both, E and
the source voltage after ωt = π, falls to zero at β. Due to the absence of
current T1 and T3 turn-off. Motor terminal voltage is now equal to its
induced voltage E. When thyristors T2 and T4 are fired at (π + α), next cycle
of the motor terminal voltage va starts.
In continuous conduction mode of Single Phase Fully Controlled Rectifier
Control of DC Motor, a positive current flows through the motor, and
T2 and T4 are in conduction just before α. Application of gate pulses turns
on forward biased thyristors T1 and T3 at α. Conduction of T1 and T3 reverse
biases T2 and T4 and turns them off. A cycle of va is completed when T2 and
T4 are turned-on at (π + α) causing turn-off of T1 and T3.
Since armature current is is not perfect dc, the motor torque fluctuates.
Since torque fluctuates at a frequency of 100 Hz, motor inertia is able to
filter out the fluctuations, giving nearly a constant speed and rippleless E.
Discontinuous Conduction:
• In a Single Phase Fully Controlled Rectifier Control of DC Motor terminal voltage v a,
the drive operates in two intervals (Fig. b):
• Duty interval (α ≤ ωt ≤ β) when motor is connected to the source and va = vs.
• Zero current interval (β ≤ ωt ≤ π + α) when
ia = 0 and va = E.
Drive operation is described by the following equations:
……….(1)
……….(2)
……….(3)
……….(4)
……….(5)
……….(6)
Continuous Conduction:
From Fig. (c)
……….(7)
As we know E Km ……….(8) T K I a ……….(9)
……….(10)
So speed in rad/sec is
……….(11)
The ideal no load operation is obtained when Ia = 0. When both thyristor pairs (T1,
T3) and (T2, T4) fail to fire, Ia will be zero. This will happen when E > vs throughout
the period for which firing pulses are present.
Therefore, when α < π/2, E should be greater or equal to Vm and when α > π/2, E
should be greater or equal to Vm sin ωt. Therefore, no load speeds are given by
……….(12)
……….(13)
Boundary between continuous and discontinuous conduction is
shown by dotted line
For torques less than rated, a low power drive mainly operates in discontinuous
conduction. In continuous conduction, the speed-torque characteristics are parallel
straight lines, whose slope, according to eq.(11), depends on the armature circuit
resistance Ra
In continuous conduction, for a given α, any increase in torque causes ω m and E to
drop so that Ia and T can increase. Average terminal voltage Va remains constant. In
discontinuous conduction, any increase in torque and accompanied increase in
Ia causes β to increase and Va to drop. Consequently, speed drops by a larger
amount.
Maximum average terminal voltage (2Vm/π) is chosen equal to the rated motor
voltage. ideal no load speed of the motor when fed by a perfect direct voltage of
rated value will then be (2Vm/πK).
(d)
Quadrant operation for single phase fully
controlled rectifier
• The drive operates in quadrants I (forward motoring) and IV (reverse regenerative
braking).
• These operations can be explained as follows:
• From Eq. (11), under the assumption of continuous conduction, dc output voltage
of rectifier varies with α as shown in Fig.(a).
• When working in quadrant I, ωm is positive and α ≤ 90°; and polarities of Va and E
are shown in Fig. (b).
• For positive Ia this causes rectifier to deliver power and the motor to consume it,
thus giving forward motoring. Polarities of E, Ia and Va for quadrant IV operation
are shown in Fig. (c).
• E has reversed due to reversal of ωm. Since Ia is still in
same direction, machine is working as a generator
producing braking torque. Further due to α > 90°,
Va is negative, suggesting that the rectifier now takes
power from dc terminals and transfers it to ac mains.
• This operation of rectifier is called inversion and the
rectifier is said to operate as an inverter. Since
generated power is supplied to the source in this
operation, it is regenerative braking.
Application
• Two quadrant operation capability of
the drive can be utilized only with overhauling
loads or other active loads which can drive the
motor in reverse direction.
• In a normal two quadrant operation of a
motor one needs forward motoring (quadrant
I) and forward braking (quadrant II) which
cannot be provided by the drive of Fig. (a).
